Liquid crystal displays (LCDs) generally display images by transmitting or blocking light through the action of liquid crystals. LCDs have been used in a variety of computing displays and devices, including notebook computers, desktop computers, tablet computing devices, mobile phones (including smart phones) automobile in-cabin displays, on appliances, as televisions, and so on. LCDs often use an active matrix to drive liquid crystals in a pixel region. In some LCDs, a thin-film transistor (TFT) is used as a switching element in the active matrix.
FIG. 1 illustrates a perspective view of a sample electronic device, such as a tablet computer. The electronic device includes a touch screen display 100 enclosed by a housing 138. The touch screen display 100 incorporates a touch panel 102 (also referred to as a touch-sensing module) and an LCD, although alternative embodiments may employ an OLED layer instead of an LCD. The LCD is not shown in FIG. 1. Although a tablet computing device is illustrated in FIG. 1, it should be appreciated that the discussion herein may be equally applicable to any display and/or electronic device incorporating a display, such as a smart phone or other mobile phone, notebook or laptop computer, all-in-one computing device, personal digital assistant, multimedia player, e-book reader, touch-sensitive display for an automobile or appliance, GPS device, and so on.
FIG. 2A illustrates a simplified cross-sectional view of a sample tablet computing device shown in FIG. 1 (the cross-section is shown by arrows 2-2 in FIG. 1). LCD 204 includes an array of pixels and an array of TFTs associated with the pixels. The LCD 204 may be coupled to the touch panel 102 by an optically clear adhesive (OCA) 206 or may have an air gap from the touch panel. In some embodiments, the LCD 204 may be adjacent to, or co-planar with, the touch panel (also referred to herein as a touch-sensing module).
The LCD 204 may suffer from display noise, which may cross-couple to the integrated capacitive touch sensing subsystem of the tablet computing device or other electronic device. For example, the capacitance between the gate and drain of a TFT for a given display pixel may vary with a voltage between a gate and a drain, which depends upon the image being displayed. Other factors can affect the capacitance between the gate and drain of a TFT such as changes in the gate voltage, the display temperature, and the like. The change in the gate and drain parasitic capacitance acts like a display noise and may interfere with detection of touches or other capacitive sense events during operation. The display noise in the LCD 204 may be sensitive to a finger touching the touch panel 102. The impact of finger touches may vary with the user and/or environment in which the device is used. For example, a human finger may be relatively cold or warm. The device 100 may be used in a hot and humid environment, or a cold and dry environment, all of which may change the sensitivity of the LCD to display noise.